This disclosure is generally directed to methods and systems for heating in an enclosed space. In particular, the disclosure is directed to a method and system for preheating ladles having an enclosed space for use with molten metal.
In the metal industry, refractory lined ladles are commonly used to transport or store molten metal prior to further processing. The ladles are heated prior to use, typically referred to as ladle preheating, to minimize cooling of the molten product. Normally, the ladles are heated by combustion systems whereby fuel is fired to generate combustion heat in the cavity of the ladle. In practical operation, the lip of the ladle is normally covered with solidified chunks of metal and other types of slag from use, which forms an inevitable gap between the lid of the ladle and the ladle rim. The gap may be substantial, wherein the gap may extend up to a few inches or more at certain locations around the lip of the ladle. Though it is common practice to use this gap as an exhaust vent for the combustion products, air entrainment through the gap into the cavity of the ladle may occur when the gap becomes too large. The cooler entrained ambient air reduces the energy efficiency of the heating system. It also increases the NO production especially when an high temperature method, such as oxy-fuel combustion, is used. This entrainment of cool air becomes a greater problem due to the buoyancy effect when the gap orients vertically. Some prior art patents have attempted to overcome the energy loss and NO production by sealing the container to reduce the air infiltration through the gap. However, in practice, these methods of sealing are difficult to achieve and maintain because the solidified chunks of metal and other types of slag covering the lip of the ladle disrupt the seal and/or cause damage to the sealing surface on the ladle heating apparatus.
Various methods of sealing of the ladle rim or the ladle heating furnace are claimed in U.S. patent application Ser. Nos. 1,057,905; 4,223,873; 4,229,211 (which is a continuation-in-part of U.S. Pat. No. 4,223,873); U.S. Pat. Nos. 4,386,907; 4,364,729; and 5,540,752, each of which are incorporated by reference in their entirety. In practice, these methods of sealing suffer from the drawback that they are difficult to achieve because the lip of the ladle is normally covered with solidified chunks of metal and other types of slag which disrupt the seal and/or cause damage to the sealing surface on the ladle heating apparatus.
Historically, the air-fuel combustion has been the conventional technology used in nearly all industrial heating processes. In general, it is easier to maintain positive pressure and minimize air leakage in air-fuel combustion due to the much larger volume of flue gases, as compared to oxy-fuel combustion, However, air-fuel combustion is generally inefficient without heat recovery methods. To improve the energy efficiency, recuperative or regenerative air-fuel combustion has been widely adapted in various melting furnaces. U.S. Pat. Nos. 1,057,905; 4,223,873; 4,229,211; 4,386,907; 4,364,729; 4,359,209; 4,718,643, Korean Patent KR2000004271A, and Belgium Patent BE901913A, each of which is hereby incorporated by reference in their entirety, disclose using recuperative or regenerative air-fuel combustion system for the ladle preheating. However, these recuperative or regenerative air-fuel combustion systems are complicated, expensive, and commonly require frequent maintenance.
U.S. Pat. No. 1,057,905 introduces an apparatus for drying and heating the ladle by using the air fuel combustion. To seal the ladle rim and preheat the combustion air, the method disclosed in U.S. Pat. No. 1,057,905 inverts the ladle and places the ladle over the furnace. By doing so, flue gas is prevented from escaping and no ambient air is entrained from the rim of the ladle. Drawbacks of the U.S. Pat. No. 1,057,905 include the following: (1) The ladle must be inverted which is an unacceptable posture for large steel mill ladles; (2) In actual operation, it is difficult to keep the rim of the ladle clean or make solidified metal and slag evenly distributed along the rim. These limitations make it difficult to ensure the seal connection when docking the rough rim of the ladle with the furnace.
U.S. Pat. No. 4,223,873 discloses a recuperative air-fuel flame ladle heating method and apparatus. To prevent excessive leakage between the interior of the ladle and the outside atmosphere, a circular seal comprising a ceramic fiber compaction material is added between the rim of the ladle and the opening of the casing of the heat exchanger system and the burner system. This method utilizes additional material and does not prevent buildup of material along the lip of the ladle thereby having limited use in sealing the ladle.
U.S. Pat. No. 4,229,211 discloses a ladle heated by a direct air-fuel flame. A seal is applied to the rim of the ladle and directing air through a heat exchanger and to the ladle, mixing fuel with the air and igniting the mixture and directing the flame in to the ladle chamber, and exhausting the gases of combustion from the ladle chamber back through the heat exchanger. The seal applied to the rim of the ladle comprises a network of refractory fiber modules mounted in a common plane. Each module comprises a rectangular block formed of a web of refractory fibers in an accordion folded arrangement, and the modules are mounted with their folded edges exposed, and with the folds of each module extending at right angles with respect to the folds of the adjacent modules. The method of U.S. Pat. No. 4,229,211 suffers from the drawback that it cannot be properly utilized in many cases without clearing the rim of the ladle to remove solidified metal and slag, and the heat wall must be relined frequently due to damage to the compressible lining of the seal assembly of the heater by docking with the rough rim of the ladle.
U.S. Pat. No. 4,386,907 discloses a sealing means for preheating the ladle with the stopper rod opening. The method of the U.S. Pat. No. 4,386,907 suffers from the drawback that it requires additional materials and structures for sealing and does not prevent buildup material along the lip of the ladle, thereby having limited use in sealing the ladle.
U.S. Pat. No. 4,364,729 discloses a ladle heating system with air seal and heat shield. The ladle is heated by a direct flame, by applying a lid to the rim of the ladle and directing an air stream through a heat exchanger and through the lid to the ladle, mixing fuel with the air and igniting the mixture and directing the flame into the ladle chamber, and exhausting the gases of combustion from the ladle chamber through the lid and heat exchanger. A heat shield is mounted adjacent the lid and is sized and shaped to telescopically receive the rim of the ladle, and a ring of air is moved between the heat shield and the rim of the ladle to an air pickup ring. The ring of air blocks the gases escaping from inside the ladle through any openings between the ladle rim and the lid, the heat shield blocks heat radiation from any such openings. The U.S. Pat. No. 4,364,729 suffers from the drawback that the process requires an air shield to prevent gases from escaping, but does not prevent ingress of air into the ladle.
U.S. Pat. No. 5,540,752 discloses a process and apparatus for recovering a non-ferrous metal in molten coherent form from the scrap and dross. The scrap and dross are introduced through a sealable rotary furnace. A seal means of the furnace is described to exclude the leakage of ambient air to the rotary furnace. The U.S. Pat. No. 5,540,752 discloses a seal structure that does not prevent the buildup of material on the lip of the ladle, thereby limiting to use in sealing the ladle.
U.S. Pat. No. 2,294,168, which is hereby incorporated by reference in its entirety, discloses a special type of gas burner, designed for the heating of the interior surfaces of vessels of circular cross section, particularly, of such vessels as hot metal ladles. The said gas burner comprises an air conduit extending into the vessel and a gas feeding pipe within the conduit. The tip of the said burner includes a Venturi opening to direct hot combustion products upwardly around the air conduit to preheat air provided through the air conduit. The Venturi opening form of the burner tip creates pressure differentials in the vessel for swirling the hot products of combustion uniformly against the walls of the vessel being heated. The U.S. Pat. No. 2,294,168 suffers from the drawback that burner structures extend into the vessel creating greater exposure for the components, and thereby increasing the maintenance costs and/or material costs for the burner.
U.S. Pat. No. 4,359,209 discloses a ladle preheat apparatus and method which utilizes recuperation but which totally eliminates the need to create any seal between the rim of the ladle and the ladle lid. The said ladle preheat method comprises an outer casing defining an opening for receiving the combustion products from the ladle. The opening is dimensioned so as to form a dilution air space about the ladle which permits ambient air to be drawn in therearound and to mix with the combustion products. The U.S. Pat. No. 4,359,209 suffers from the drawbacks that ambient air is drawn in to the area of combustion completely surrounding or partially surrounding the ladle. The seal means by the air space is to prevent the combustion products from exhausting to the ambient; however, the ambient air may be drawn into the ladle and reduce the flame temperature and potentially increase NOx production.
U.S. Pat. No. 3,412,986, which is hereby incorporated by reference in its entirety, discloses a double-ended oxy-fuel burner which is intended especially for heating ladles (torpedo ladles) that are used in iron and steel works for transporting molten metal from a blast furnace to another location. The said burner is used to preheat the ladle or keep the ladle from cooling between successive loads of metal. The burner is proportioned to the ladle and has oppositely directed orifices for producing long flames and for projecting them in opposite directions. The burner is constructed with water cooling jackets extending all the way to the ends of the tips so that the burner can withstand the high temperature within the ladle. The said burner includes two concentric tubes. The inner tube is for fuel and the outer is for oxygen. At the tips of the burner, the fuel is introduced to the ladle through one tube. The oxygen stream is introduced to the ladle through multiple orifices which are distributed annularly along the central fuel tube. The U.S. Pat. No. 3,412,986 suffers from the drawback that the burner includes significant structures that extend into the vessel, thereby increasing maintenance costs and/or material costs for the burner.
U.S. Pat. No. 4,718,643, which is hereby incorporated by reference in its entirety, discloses a method and apparatus for rapid high temperature ladle preheating utilizing an optimized heating cycle by involving oxygen and combustion air preheated by recuperation in the fuel burning process. Controlled oxygen flow directed into the process is used to increase the heat input during the initial preheating phase and to insure efficiency of the system during the soaking phase of ladle preheating. The disclosed ladle lid comprises a partially open refractory ring for docking a portion of a ladle rim having a significant protrusion caused by a local accumulation of solidified metal or slag. The flame from the burner located in the ladle lid transfers heat to the interior of the ladle and the flue gases are discharged from the ladle into the exhaust port. As the ladle is partially sealed, some ambient air is drawn into the exhaust port around the lip of the ladle. The drawing in of ambient air creates an effective seal of the ladle, which prevents a chimney effect which would draw hot gases out of the ladle if the ladle is not positioned very closely to the lid of the ladle. Therefore, the U.S. Pat. No. 4,718,643 permits the handling of rough-edged ladles without destroying the seal. U.S. Pat. No. 4,718,643 suffers from the drawback that, as the ladle is partially sealed, exterior ambient air is drawn through the opening toward the interior of the ladle, reducing the flame temperature and potentially increasing NOx production.
Korean Patent No. KR20040056882, which is hereby incorporated by reference in its entirety, discloses a multi-hole nozzle burner for heating a ladle is provided to reduce NOx emissions and save fuel by reducing heating time. The multi-hole nozzle burner is installed at the center of an upper ladle cover, and has a triple ring-shaped structure, which includes an ignition air nozzle having a single jet, fuel nozzles surrounding the ignition air nozzle and having multiple jets, and combustion air nozzles enclosing the fuel nozzles and having multiple jets. The KR20040056882 system suffers from the drawback that the combustion utilizes only air-fuel combustion, which has reduced flame temperature and increased NOx production.
Korean Patent No. KR20000042710A discloses a regenerative burner is provided to save the energy. Combustion gas is sufficiently circulated inside the ladle by using a high velocity nozzle-mixing type burner. The K20000042710A system suffers from the drawback that the combustion only utilizes air-fuel combustion, which has reduced flame temperature and increased NOx production.
U.S. Patent Application Publication US20090220900, which is a divisional application of U.S. Pat. No. 7,549,858, both of which are hereby incorporated by reference in their entirety, discloses heating provided by a burner that combusts a hydrocarbon fuel which can be provided at a sequence of different heat transfer rates by adjusting the total oxygen concentration of oxidant streams fed to the burner. However, the impact of the burner on the air ingress into the heating vessel is not considered by the US20090220900.
Belgium Patent Publication No. BE901913A, which is hereby incorporated by reference in its entirety, discloses a ladle heating system that involves the use of a fixed hood to which the ladles are moved into position. The fixed hood is provided with a burner in its upper section and a waste gas outlet in its lower section. Burnt gases are circulated in the ladle at relatively high speed to ensure uniform heating. Heat in the waste gases is recovered by means of either a recuperative or a regenerative system enabling combustion air and possibly combustion gas to be preheated. The BE901913A Publication suffers from the drawback that air ingress is not prevented, which reduces flame temperature and increases NOx production.
Flameless combustion is a combustion technology wherein the reactants for combustion are highly diluted before they mix and react. Flameless combustion is typically utilized when NOx control is desired. The reactants usually are diluted by entraining combustion products before the combustion reactions occur. This mode of combustion typically occurs when the oxidizing gas is diluted to a level below 17% oxygen, wherein the flame front disappears and the fuel oxidizes in a flameless fashion. The key to this technology is the maintenance of the furnace temperature above the auto-ignition temperature of the fuel or the use of a highly-robust flame stabilizer. This type of combustion is usually characterized by high jet momentum and a large combustion volume. Flameless combustion can alternatively be referred to as spacious or distributed combustion.
There is a need in the metals industries for heating methods and systems for enclosed spaces, such as ladle and furnace heating that reduce or eliminate air ingress. These needs are addressed by the embodiments of the present invention as described below and defined by the claims that follow.